This invention relates generally to gas springs and more particularly to a as spring constructed to reduce the net force across a piston rod assembly of the spring during at least a portion of the stroke of the piston rod assembly.
Gas springs are well known and have been used in dies of presses for sheet metal stamping operations. Conventional gas springs have a gas chamber which receives a pressurized gas that provides a force on a piston and piston rod of the gas spring to bias them to an extended position and to resist the movement of the piston and piston rod from their extended position to a retracted position. Various retainers and seals are provided in the gas spring to retain the piston and piston rod within a casing of the gas spring and to prevent leakage of the pressurized gas from the gas chamber.
Typically a plurality of gas springs yieldably urge a binder ring or clamp ring of the die assembly into engagement with a sheet metal workpiece as the dies are closed by the press to cold form the workpiece. Typically the press has a mechanical drive mechanism with a toggle linkage or a crank and meshed gears to advance and retract a press ram relative to a bed to open and close the dies. In presses with mechanical drive mechanisms the mechanical advantage of the mechanism varies depending on the position of the ram and increases dramatically as the ram approaches the fully advanced or extended position to completely close the dies and form the workpiece. As the ram advances from its middle stroke position, its mechanical advantage and the maximum instantaneous force produced by the press are relatively low.
When the ram of the press is in its middle stroke position, the piston and rod of the gas spring are typically in their fully extended position and they are initially moved toward their retracted position well before the ram reaches its fully extended position. Consequently, the gas springs initially apply a relatively large force or load to the drive mechanism of the press relative to the maximum force the press is capable of producing during its initial movement of the ram from its fully retracted position toward its advanced position closing the dies. Moreover, since the mechanical drive mechanism of the press has already begun moving and accelerating the ram and the die half attached thereto before it begins to actuate and overcome the resistance produced by the gas springs, the initial impact or actuation of the gas springs creates an impulse force spike of high magnitude and short duration on the drive mechanism of the press which over time can significantly shorten the useful life of the press, significantly increase the press maintenance and repair cost, and in some instances even rapidly seriously damage the drive mechanism of the press.
Similarly, during opening of the closed dies after a workpiece has been formed, the gas springs apply significant force to the drive mechanism of the press which is suddenly terminated when their piston rods become fully extended which produces a sudden change in the load on the drive mechanism. This rapid force change can also damage the press and vibration or bouncing of the binder ring is caused at this same time by the inertia of the die component. This gas spring will reduce the magnitude of the bounce after a part is made.
A gas spring having a pair of sealing surfaces which, during a portion of the stroke of a piston rod assembly of the gas spring, define a gas tight seal between them to define a secondary gas chamber separate from a main gas chamber of the gas spring. Each gas chamber contains a gas under pressure and provides a force acting on the piston rod assembly in generally opposed directions to reduce the net force on the piston rod assembly. Desirably, when the piston rod assembly is near its fully extended position the sealing surfaces provide the gas tight seal and the differential force across the piston produces only a small net force in the direction tending to move the piston rod assembly to its extended position. Thus, a greatly reduced force is required to initially move the piston rod assembly from its extended position toward its retracted position. This greatly reduces the initial resistance to movement of the gas spring and hence the impulse or impact force imparted to a press ram upon initial actuation of the piston rod of the gas spring. After the gas tight seal is terminated, the gas spring functions generally as a conventional gas spring with a single gas chamber providing a pressurized gas acting on the piston and biasing the piston rod assembly towards its extended position against the force of the press ram displacing it.
Desirably, during the return stroke and after the seal is initially formed between the sealing surfaces, the gas in the secondary gas chamber is compressed by further movement of the piston rod assembly toward its extended position producing a higher pressure of gas in the secondary gas chamber compared to the main gas chamber. The gas in the secondary gas chamber acts on a considerably smaller surface area than the gas in the main gas chamber and therefore, this higher-pressure gas in the secondary gas chamber provides an increased force to reduce the net force on the piston rod assembly.
In one embodiment, a first sealing surface is preferably formed in a retainer which has a stop surface to maintain the piston rod assembly within a casing of the gas spring. The second sealing surface is defined by a sleeve carried by the piston rod assembly, and may be defined by an annular sealing member, such as an O-ring, carried by the sleeve to provide the gas tight seal between the sealing surfaces. In another embodiment, the first sealing surface is defined by the interior surface of the casing of the gas spring. As still another alternative, the second sealing surface may be defined integrally with the piston and/or the piston rod. In whatever form, the sealing surfaces provide a gas tight seal, to define the secondary gas chamber separate from the main gas chamber, which is initiated at a point in the piston rod assembly stroke between its extended and retracted positions and continues to the extended position of the assembly. Desirably, the secondary gas chamber is relatively small in comparison to the first gas chamber to reduce the volume of gas which is compressed after the gas tight seal is initiated to limit the increase in temperature of the gas spring.
In another embodiment, a second pair of sealing surfaces are provided to define, when the sealing surfaces are engaged with each other, separate gas chambers to reduce the speed of the piston rod assembly as it moves adjacent to and towards its fully extended position. This slower movement of the piston rod assembly provides a smoother or more subtle transition from movement of the piston rod assembly toward its extended position to a stopped or static state of the piston rod assembly in its fully extended position. This reduces or eliminates the bounce of a binder or clamp ring and the formed workpiece on the binder ring after the press ram is removed from or releases from the binder ring on the piston rod in the return stroke of the piston rod assembly. Additionally, this greatly reduces the noise caused by the bouncing binder ring and workpiece and the metal to metal contact within the gas spring which limits movement of the piston rod assembly and defines the fully extended position of the piston rod assembly. Further, this reduces or eliminates misalignment of the formed workpiece relative to the binder ring, which was previously caused by the bouncing of the binder ring and workpiece, to facilitate subsequent handling of the formed workpiece.
Objects, features and advantages of this invention include providing a gas spring which requires a reduced force to initially displace a piston rod assembly from its extended position, reduces the velocity of the piston rod assembly at least when it is adjacent to and moving towards its extended position, reduces the impulse or impact force on a press when it initially engages the gas spring, reduces damage to the press and gas spring, extends the useful life of a press and gas spring, reduces noise of the press in use, reduces noise of the gas spring in use, reduces vibration and misalignment of work pieces formed by the press, reduces or eliminates bounce of a binder ring and workpiece after the return stroke of the piston rod assembly, maintains a relatively low gas spring operating temperature, is easy to service and repair, is durable, reliable, of relatively simple design and economical manufacture and assembly and has a long and useful in service life.